1. Field of the Invention
The present invention relates to Enhanced General Packet Radio Service Networks; and more particularly, link adaptation in such networks.
2. Description of Related Art
The enhancements that give Enhanced General Packet Radio Service (EGPRS) its name were designed for one purpose: squeezing more data carrying capacity out of the Global System for Mobile Communication (GSM) air interface.
GSM and General Packet Radio Service (GPRS) networks both employ Gaussian filtered Minimum Shift Keying (GMSK) to transmit data between mobile stations and base station subsystems. Because of its narrow bandwidth, robustness to fading and phase noise, and relatively low implementation complexity, GMSK is an attractive modulation scheme for digital cellular networks.
One of the reasons for GMSK's robustness is its low modulation level: GMSK modulation carries a single bit per channel symbol. To exploit variations in channel quality, GPRS networks employ four different airlink coding schemes, each adding a different level of protection against symbol errors. An airlink coding scheme is an error correction code that generates a total of n encoded bits for each k information bits, and therefore, has a coding rate k/n. CS-1 is GPRS's lowest rate code. CS-4 is GPRS's highest rate code. In the set containing codes {CS-1, CS-2, CS-3}, CS-3 is the highest rate code, CS-1 is the lowest rate code.
One coding scheme is stronger than another if it is capable of correcting more channel errors per block. CS-1 is stronger than CS-2, CS-3 and CS-4. Similarly, CS-2 is weaker than CS-1, since CS-2 is capable of correcting fewer channel errors than CS-1.
GPRS's lowest rate code, CS-1, employs a relatively high number of redundancy bits and offers a maximum logical link control (LLC)-layer throughput of 8 kbps/timeslot. The high level of redundancy present in blocks encoded using CS-1 ensures that mobile stations at the fringes of a cell, where C/I levels are typically lowest, are able to send and receive data. In contrast, the highest rate code, CS-4, offers maximum LLC-layer throughputs of 20 kbps/timeslot. Because of the small number of redundancy bits added to each block encoded with CS-4, however, airlink errors can be detected, but not corrected. As a result, CS-4 offers the best airlink performance at relatively high C/I ratios. The remaining two GPRS coding schemes offer maximum LLC-layer throughputs of 12 kbps/timeslot (CS-2) and 14.4 kbps/timeslot (CS-3).
In EGPRS and GPRS networks, link layer packets are transferred over the air interface using unidirectional, physical layer connections known as Temporary Block Flows (TBFs). When the wireless data network wishes to transmit link layer data packets to a mobile station, a downlink Temporary Block Flow is established. During the establishment of a downlink TBF, the network assigns the mobile one or more timeslots that will be used for the transfer as well as other allocation parameters such as a temporary address. Mobile stations wishing to transmit link layer data packets to the network must first request establishment of an uplink TBF. If the network has sufficient uplink bandwidth to satisfy the request, the network establishes an uplink Temporary Block Flow, and tells the mobile which uplink timeslots it is permitted to use and assigns it temporary addresses on each timeslot known as Uplink State Flags. TBFs are maintained only as long as there are blocks to be transferred.
For backward compatibility, EGPRS employs the same slot structure and block-oriented data transfer over the air interface as its predecessor, GPRS. To enable higher data carrying capacities over the air interface, EGPRS employs 8-PSK modulation in addition to GMSK. 8-PSK modulation has three times the bit-per-symbol density of GMSK, resulting in potentially higher airlink throughputs.
GPRS networks have a single degree of freedom for selecting the coding used to transmit blocks over the air interface—the data encoding scheme to use for each radio block. In contrast, EGPRS networks enjoy two degrees of freedom: selection of a modulation scheme—GMSK or 8-PSK—and selection of a data encoding scheme. A total of nine different Modulation and Coding Schemes (MCSs) are defined by the EGPRS system specifications. Table 1 gives a summary of the maximum LLC-layer throughputs achieved by each of EGPRS' modulation and coding schemes.
TABLE 1EGPRS's modulation and coding schemes.Maximum LLCModulationlayerSchemeFamilySchemethroughputMCS-1CGMSK8.8kbps/timeslotMCS-2BGMSK11.2kbps/timeslotMCS-3AGMSK14.8kbps/timeslotMCS-4CGMSK17.6kbps/timeslotMCS-5B8-PSK22.4kbps/timeslotMCS-6A8-PSK29.6kbps/timeslotMCS-7B8-PSK44.8kbps/timeslotMCS-8A8-PSK54.4kbps/timeslotMCS-9A8-PSK59.2kbps/timeslot
The GMSK-based modulation and coding schemes used by EGPRS (MCS-1 through MCS-4) are defined to provide backward compatibility with GPRS networks. The data encoding schemes used to send downlink blocks encoded with MCS-1 through MCS-4, for example, are encoded in such a way that GPRS mobiles are also able to decode the Uplink State Flag (USF) embedded in each downlink block. Hence, GPRS and EGPRS mobiles can have active Temporary Block Flows (TBFs) on the same PDCH simultaneously.
EGPRS networks have introduced an RLC/MAC resegmentation scheme. The scheme allows errored RLC/MAC blocks to be resegmented into an integral number of RLC/MAC blocks that are re-transmitted using a stronger modulation and coding scheme. To implement the scheme, EGPRS's nine modulation and coding schemes are divided into three coding families. Resegmentation can only be done within a family. The scheme enables seamless switching between coding schemes.
Due to its higher-level modulation, under similar channel conditions, 8-PSK modulation is not as robust as GMSK. To overcome the loss in performance introduced by 8-PSK modulation and to help milk more capacity from the airlink, EGPRS networks have introduced another substantial improvement to the radio link control layer: support of incremental redundancy.
In systems employing incremental redundancy, retransmissions of errored blocks carry additional redundancy bits to help the receiver correctly decode the block. In this manner, additional redundancy is added only when needed, potentially increasing the throughput of the airlink. Fields in the header identify the sequence number of the block and the redundancy scheme applied by the transmitter. The receiver can jointly decode multiple versions of the same block—so-called soft combining—improving receiver performance.
An EGPRS link adaptation algorithm must select the proper modulation and coding scheme to use on the airlink. It must also determine whether blocks should be resegmented before retransmission. The algorithm must adapt to changes in airlink quality.